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Advances in 3D NLTE Effects on Stellar Spectra: A Comprehensive Study

This study explores the impact of 3D non-local thermodynamic equilibrium (NLTE) effects on stellar abundance determination, focusing on the continuum spectra. Key parameters such as effective temperature, surface gravity, and chemical abundances are analyzed to compute absolute fluxes. Updated opacities and model atmosphere codes are employed, highlighting new tools for recalculating chemical compositions. The results show that 3D effects are modest, presenting a promising approach to reproduce solar spectrum characteristics in specific ultraviolet regions, with minor adjustments to elemental abundances.

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Advances in 3D NLTE Effects on Stellar Spectra: A Comprehensive Study

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  1. An Observational take on 3D + NLTE effects C. Allende Prieto MSSL, University College London

  2. Chemical Abundances from the Continuum C. Allende Prieto MSSL, University College London

  3. Not alone … Lars Koesterke (TACC, Austin) David Lambert (McDonald Observatory) Martin Asplund (MPA) Ivan Hubeny (Univ. of Arizona) Sultana Nahar (Ohio State) Manuel Bautista (IVIC, Caracas)

  4. Spectroscopic Abundance Determination • Input parameters for a stellar atmosphere: Teff, logg, [M/H] but in general (H,He,Li,Be,B,C, …) • Typically Teff is determined from the continuum, sometimes logg, but all other parameters from lines • For most stars: continuum shaped by H, H- in optical/IR, but other elements become important in the UV

  5. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosphere code

  6. Opacities: an update • Line opacities: Kurucz compilations • Continuum opacities: TOPBASE for C, Mg, Al, Si, Ca

  7. MgI Σσi ni / N = Σσigi exp(-Ei /K/T)

  8. Energy adjustment

  9. Opacities: an update • Line opacities: Kurucz compilations • Continuum opacities: TOPBASE for C, N, O, Mg, Al, Si, Ca • Continuum opacities: Iron Project for FeI (Bautista 1996,1997), FeII (Nahar & Pradhan 1996, 1999)

  10. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosphere code

  11. Synspec upgrades • Partition functions from Irwin (1981) • Chemical equilibrium: Subroutines from Uffe Jørgensen (molecular data from Tsuji 1973, not partition functions) • Electron density recomputed for consistency

  12. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosp. code

  13. Model atmosphere code • Atlas9 newodf (ξ=2 km/s) grid • Linux port of Atlas9 (Sbordone & Bonifacio 2006) • Newly developed tools for changing the chemical composition and recomputing models

  14. Important elements • Electron donors: Fe, Mg, Si, Ca • Line opacity: Fe-peak, O (OH), C (CH, CO), Mg, H • Bound-free free-free opacity (H, H-, Fe, Mg, Si, Al)

  15. The “trick” • Keep the T-ρ structure • Recompute Chemical equilibrium and recalculate the electron density and the radiative transfer for each composition

  16. 0.2 dex

  17. He – consistent model -0.2 +0.2

  18. A poor trick!

  19. Synspec vs Atlas: continuum

  20. Synspec vs Atlas: continuum

  21. But a small effect, anyway + 5 K

  22. Metals

  23. Mg

  24. Are the continuum opacities that different?

  25. M/H, Mg/H, C/H, O/H • 3x3x3x3 grid • Interpolation to find the best match to the SOLSTICE-UARS/SUSIM solar spectrum • The best match takes place for [M/H] = - 0.18 [Mg/H]= +0.17 [C/H] = +0.12 [O/H] = +0.07

  26. 3D effects? • New 3D code Assεtt (Koesterke 2007) • Handles arbitrarily complex opacities • Electron and Rayleigh (H) scattering • High accuracy interpolations (cubic Bezier splines) Too large a job for the full spectrum with 100 snapshots and δλ ~ Doppler

  27. 3D

  28. 2000 2000

  29. Summary • The UV continuum contains valuable abundance information • Computing absolute fluxes requires a consistent model atmosphere calculation • 3D effects appear to be modest in the continua (~ 5% at most) • Our preliminary calculations can reproduce fairly well the solar spectrum in the regions 200-260 nm and 300-400 nm with modest abundance adjustments, but too much absorption in the region around 280 nm

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